FREETOWN/MONROVIA, Aug 4 (Reuters) - Hundreds of troops deployed in Sierra Leone and Liberia on Monday to quarantine communities hit by the deadly Ebola virus, as the death toll from the worst-ever outbreak reached 887 and three new cases were reported in Nigeria.

With healthcare systems in the West Africa nations overrun by the epidemic, the African Development Bank and World Bank said they would immediately disburse $260 million to the three countries worst affected - Sierra Leone, Liberia and Guinea.

The World Health Organization, which warned last week of catastrophic consequences if the disease were not controlled, reported 61 new deaths in the two days to Aug. 1 as the disease continues to spread.

The outbreak began in February in the forests of Guinea. The toll there continues to rise, but the epicentre has since shifted to neighbouring Liberia and Sierra Leone.

In Nigeria, where U.S. citizen Patrick Sawyer became the first person to die of the virus after arriving from Liberia in late July, the WHO reported three new cases, two of them probable and one suspected.

Nigerian authorities had said earlier on Monday that a doctor who treated Sawyer had contracted the disease. A health ministry official declined to comment on the discrepancy.

Panic among local communities, which have attacked health workers and threatened to burn down isolation wards, prompted Sierra Leone, Liberia and Guinea to announce tough measures last week, including the closure of schools and the quarantine of the remote forest region hardest hit by the disease.

Long convoys of military trucks ferried troops and medical workers on Monday to SierraLeone's far east, where the density of cases is highest. Military spokesman Colonel Michael Samoura said the operation, code-named Octopus, involved around 750 military personnel.

Troops will gather in the southeastern town of Bo before travelling to isolated communities to implement quarantines, he added. Healthcare workers will be allowed to come and go freely, and the communities will be kept supplied with food.

In neighbouring Liberia, President Ellen Johnson-Sirleaf and ministers held a crisis meeting on Sunday to discuss a series of anti-Ebola measures as police contained infected communities in the northern Lofa county.

Police were setting up checkpoints and roadblocks for key entrance and exit points to those infected communities, which nobody will be allowed to leave. Troops were deploying to badly affected areas to prepare to enforce the measures.

"The situation will probably get worse before it gets better," Liberian Information Minister Lewis Brown told Reuters. "We are over-stretched. We need support; we need resources; we need workers."

WHO SEEKING FUNDS

WHO chief Margaret Chan warned regional leaders on Friday that Ebola was outpacing their efforts to contain it and pledged to organise a $100 million international response to bring the outbreak under control. U.S. officials and multilateral agencies were due to discuss the emergency at a three-day U.S.-Africa summit in Washington, which started on Monday.

A Reuters witness in the Liberian capital Monrovia said several clinics were spontaneously closing their doors because doctors were too afraid to treat patients. More than 60 doctors have already died of Ebola, hampering efforts to control the outbreak.

Health workers in Liberia say they are overwhelmed by the number of cases, a condition exacerbated by the departure of some international staff following the infection of two U.S. aid workers in Liberia.

One of them, 33-year-old Dr. Kent Brantly, was receiving treatment in an isolation ward at the Emory University Hospital in Atlanta after being flown back to the United States for treatment on Saturday.

A medical flight carrying the second American left Liberia at 0111 GMT on Tuesday (9:11 p.m. EDT Monday), a Reuters witness said. Missionary Nancy Writebol, 59, was expected to arrive in the United States by midday, when she would also be taken to Emory for treatment, according to Samaritan's Purse.

A spokesperson for the charity confirmed a CNN report that Brantly and Writebol were both treated with an experimental serum developed by biotech firm Mapp Biopharmaceutical Inc while in Liberia, apparently showing initial signs of an improvement.

Medical charity Medecins Sans Frontieres (MSF), which normally spearheads the fight against Ebola, has only a small team in Liberia and says it does not have the capacity to increase it.

The normally bustling streets of Sierra Leone's capital Freetown were eerily quiet on Monday after President Ernest Bai Koromo called on residents to stay home and pray, a Reuters reporter said.

Ambulances and police vehicles lined the streets, while radio stations played interviews with health ministry officials and a musical jingle informing the local population of symptoms.

Highly contagious, the deadliest strain of the Ebola virus can kill up to 90 percent of those infected, though in the current outbreak the rate is running around 55 percent. Symptoms initially include muscle pains and joint aches, then worsen to vomiting, diarrhoea and internal and external bleeding in the final stages.

Officials seeking to bury Ebola victims faced protests at a burial site in a suburb of Monrovia this weekend and about 25 soldiers were called in to guard the site.

(Additional reporting by Daniel Flynn and Emma Farge in Dakar, Tom Miles in Geneva, Tim Cocks in Nigeria and Lesley Wroughton in Washington; Writing by Emma Farge and Daniel Flynn; Editing by Will Waterman, Larry King and Lisa Shumaker)